Enable doc test on ci

Author: k-nasa

.github/workflows/ci.yml

@@ -65,6 +65,12 @@ jobs:
         command: test
         args: --all --features "unstable attributes"
 
+    - name: documentation test
+      uses: actions-rs/cargo@v1
+      with:
+        command: test
+        args: --doc --features "unstable attributes"
+
   check_fmt_and_docs:
     name: Checking fmt and docs
     runs-on: ubuntu-latest